drm/i915: Remove unused intel_ring_cacheline_align

The last use of intel_ring_cacheline_align() was removed in 2017 by
commit afa8ce5b ("drm/i915: Nuke legacy flip queueing code")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Tvrtko Ursulin <tursulin@igalia.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241227113754.25871-3-tursulin@igalia.com